Updating Core Data object ID URIs on store migrations

I found out recently that Core Data object IDs can change. This usually happens if you migrate a store, such during a heavyweight migration (lightweight migrations don’t count, as they mutate in place). If you were like me and persisting object IDs, then you’ll have to update those URIs, or you will have a very bad day trying to fetch objects from these IDs.

The easiest way to do this is to lean on NSMigrationManager, because it has both the source and destination model versions, and can get the destination object version from the source version. If you’re manually doing the migration for model upgrades, you easily have it already. If you want to update it from a NSEntityMigrationPolicy, then the easiest option is to stash the NSMigrationManager by overriding beginEntityMapping:manager:error:. You provide it the source NSManagedObjects, and out comes the destination version’s managed objects, as shown below, of which you can get the new object IDs to persist in the new version of the model:

var manager: NSMigrationManager!

private func destinationURIsFromSourceObjects(_ objects: [NSManagedObject]) -> [URL] {
    return manager.destinationInstances(forEntityMappingName: "TrackToTrack", sourceInstances: objects).map {

Note that you need to provide the entity mapping name – regardless if your entities heavily changed or not. For the Xcode mapping model generator, check the name – it’s usually something like TrackToTrack. If you need to get the managed objects from the source version’s URIs, you can use the the sourceContext property in the migration manager to get the source model version’s managed object context.

Sleeping through a decade of Cocoa: Retrospective from modernizing an old Mac app

A few years ago since I started using Macs more often, one annoying thing I dealt with was using my local music library. My usual solution was to just drag files from the file manager to a music player, but this wasn’t as nice on macOS (due to i.e. SMB latency). However, I did have a Subsonic server, which provides a nice music streaming server, complete with an API for clients to use for things like phones. Why not use this on my laptop too?

Of course, if I bought a Mac, I’m not going to put up with bad cross-platform solutions that suck everywhere, when I can instead run bad native software that sucks uniquely for my platform of choice. However, there weren’t too many clients available on Mac. Mostly all of them were unmaintained and had been abandoned in the Snow Leopard era. One of them was Submariner, and it was open-source after the developer (Rafaël Warnault) had stopped working on it. Writing my own seemed a bit daunting with no background, but what if I used the Submariner codebase, and started from there?

Now I’ve been maintaining Submariner for almost two years at this point (it even has a minimal website), adding features and mostly just focusing on modernizing the codebase. It’s been an interesting experience as my first Objective-C/Mac project. A lot of the lessons of modernizing legacy code are universally applicable, but I’ve learned a lot about the specifics of Apple platforms and how they compare. This article aims to be both a retrospective on what I had to learn, what I had to do, and the lessons I took from it, including a comparison of what the development culture is like between platforms.

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